1. Field of the Invention
The present invention relates to a vertical power component capable of withstanding a high voltage (greater than 500 V), and more specifically to the structure of the periphery of such a component.
2. Discussion of the Related Art
FIGS. 1, 2, and 3 show various ways to form the periphery of a high-voltage vertical power component to enable it to withstand high voltages.
These drawings show a triac comprising a lightly-doped substrate 1, currently on the order of from 1014 to 1015 atoms/cm3, having its upper and lower surfaces comprising P-type doped layers or regions 3 and 5. Upper layer 3 contains a heavily-doped N-type region 4 and lower layer 5 contains a heavily-doped N-type region 6 in an area substantially complementary to that taken up by region 4. An electrode A1 coats the lower surface of the component and is in contact with regions 5 and 6. An upper surface electrode A2 is in contact with region 4 and a portion of region 3. In this region 3 is also formed a heavily-doped N-type region 8 of small extent and a gate electrode G covers region 8 and a portion of region 3. Thus, whatever the biasing between electrodes A1 and A2, if a gate control is provided, the component becomes conductive. The conduction occurs from electrode A1 to electrode A2 through a vertical thyristor comprising regions 5, 1, 3, and 4, or from electrode A2 to electrode A1 through a vertical thyristor comprising regions 3, 1, 5, and 6. The thickness and the doping level of substrate 1 are calculated so that the triac, in the off state, can withstand high voltages, for example, voltages greater than from 600 to 800 volts. It should then be avoided for breakdowns to occur at the ends of the components.
FIG. 1 shows a so-called mesa peripheral structure for avoiding such breakdowns. A lateral ring-shaped trench deeper than P regions 3 and 5 is formed at the periphery of each of the two surfaces of the substrate. These trenches are filled with a passivation glass 9. In practice, trenches are initially formed on a silicon wafer between two components before dicing of the wafer into individual components. A lot of research has been carried out on such mesa-type vertical power components. If a breakdown occurs, it occurs in the areas where the PN− junctions cross isolated trenches 9. In the best conditions, that is, when the angle according to which the trenches filled with glass cross the junctions between the substrate and layers 3 and 5 is properly selected, and when the quality of the glass is optimized, a distance e1 between the edge of the component and the glassivation limit at least equal to 300 μm should be provided to obtain a breakdown voltage greater than 800 volts. This decreases by the same distance the surface area available for the power component electrodes; otherwise, for given surface areas of the electrodes, this increases the surface area of the component and thus its cost.
A specific disadvantage of mesa structures is that, given that the passivation glass never has the same thermal coefficient as silicon, the interface between glass and silicon does not age well and, in case of an incidental breakdown, if the voltage across the component exceeds the authorized limit, the component is no longer operative.
FIG. 2 shows another conventional power component periphery structure. A groove filled with a passivation glass is present on the upper surface side. The component is surrounded with a heavily-doped P-type diffused wall 12 formed from the upper and lower surfaces and the groove extends between wall 12 and P-type layer 3, substantially as shown. Thus, all voltage withstand areas are gathered on the upper surface side of the component. At the periphery of the junction between wall 12 and substrate 1, on the groove side, in the area designated with reference numeral 14, breakdowns may occur when lower electrode A1 is negative with respect to upper electrode A2 (so-called reverse breakdown); and at the periphery of the junction between substrate 1 and layer 3, on the groove side, in the area designated with reference numeral 16, breakdown voltages may occur when lower electrode A1 is positive with respect to upper electrode A2 (so-called forward breakdown).
This structure provides good results and simplifies the forming of lower electrode A1, but distance e2 between the edge of the component and the limit of electrode A2 is greater than in the previous case, for example, 350 μm to withstand a voltage greater than 800 volts. Further, as in the previous case, the interface between the silicon and the passivation glass remains an issue.
Further, this method requires a greater number of masks than that of the previous structure.
FIG. 3 shows a passivation structure in so-called “planar” technology. As in the case of FIG. 3, the structure is surrounded with a heavily-doped P-type ring-shaped wall at its periphery. To withstand the voltage, a distance is provided between the limit of P-type layer 3 and peripheral wall 20. If a breakdown occurs, it occurs in bending regions 23 of P well 3 or in region 24 of junction between P layer 5 and substrate 1.
An advantage of this structure is that a breakdown is not necessarily destructive for the component. However, this structure has the disadvantage of requiring a channel stop ring 22 at the periphery of the upper surface in region N1 between the limit of P region 3 and the limit of isolation wall 20. This results in the disadvantage of requiring a relatively high guard distance e3 between the component edge and the limit of electrode A2, for example, on the order of 370 μm to withstand a voltage greater than 800 volts.